The invention relates to a transformation controlled nitride precipitation hardening heat-treatable steel with 15-18 wt. % chromium. The steel has a combination of strength, toughness and resistance to stress crack corrosion, and can therefore be used to advantage in the chemical industry, transportation technology, power station technology, building technology, and in plastics processing.
Transformation controlled martensitically hardenable steels are known state of the art, for example, the alloy 17-5 ph with 15.4 wt. % Cr, 4.4 wt. % Ni, 0.4 wt. % Mn, 0.25 wt. % Si, 3.3 wt. % Cu, 0.3 wt. % Nb and 0.04 wt. % C, or the alloy 14-5 ph with 14 wt. % Cr, 5 wt. % Ni, 0.4 wt. % Mn, 0.25 wt. % Si, 1.6 wt. % Cu, 0.25 wt. % Nb, 1.5 [wt. %] Mo and 0.05 wt. % C. The nickel and chromium contents are balanced out here so that no, or very little, delta-ferrite arises during the austenizing.
Transformation controlled steels are strengthened by martensitic transformation and by precipitation hardening. Martensite arises by means of a quenching treatment following the austenizing, while the precipitation hardening is effected by a heat treatment of the quenched martensite. Transformation controlled steels are therefore usually first austenized, quenched, and following after this, heat treated at medium temperatures. The respective structure formation is influenced by the action of the alloying elements and the heat treatment parameters on the transformation temperatures MS, Mf and Acl. Ms is the temperature at which the transformation from austenite to martensite begins during quenching; Mf is the temperature at which the transformation of the austenite to martensite during quenching is ended, and Acl is the temperature at which austenite formation begins during heating up.
The MS temperature of the martensitically hardenable steels is sufficiently high because a large part of the austenite present during austenizing can be converted into martensite by normal cooling to room temperature. The MS temperature is furthermore affected by the grain size and the dissolved substitution elements, which facilitate precipitation hardening. The coarser the grain and the higher the proportion of dissolved alloying elements, the lower is the MS temperature.
The residual austenite after a complete austenizing followed by cooling treatment is transformable. If substitution elements are precipitated during a tempering treatment, the MS temperature of the residual austenite can increase again such that this is converted into martensite again in the following annealing treatment. To be distinguished from residual austenite is the tempering austenite which remains behind after a partial austenizing, accordingly annealing in the ferrite-austenite two-phase region and subsequent cooling treatment.
For an optimized combination of precipitation hardenability and grain size limitation, two kinds of alloying elements are added in conventional transformation controlled martensitic steels:
Nb and C for precipitation hardenability, though primarily for grain size limitation;
Cu exclusively for hardening through precipitation hardening.
Massive grain growth brings about insufficient stability of the niobium carbides at temperatures above 1,050xc2x0 C., so that the austenizing at these temperatures is limited. Maximum precipitation hardenability is attained at temperatures around 450-500xc2x0 C. However, a tempering treatment at these temperatures results in very low ductilities and in particular a very low resistance against stress crack corrosion.
Tempering austenite, in contrast to residual austenite, has a very favorable effect on ductility (toughness) and stress crack corrosion resistance. It has the more favorable effects on these properties, the finer the preceding (former) austenite grain was. Ductility is well increased by means of a double austenizing, the second austenizing at lower austenizing temperatures serving, not only for grain refining (normalizing) but also for a limited precipitation of niobium carbides which, together with the grain refining, further increases the MS temperature. Tempering austenite is formed at temperatures between 550 and 650xc2x0 C., a maximum content of tempering austenite being attained at temperatures around 600xc2x0 C.
An increased proportion of tempering austenite has a favorable effect on strength and stress crack corrosion resistance. On the other hand, especially at elevated carbon contents, the formation of tempering austenite during a tempering treatment in the region between 550 and 650xc2x0 C. is associated with a sensitization of the austenite. By this is understood a worsening of the corrosion resistance (particularly to intercrystalline corrosion) due to grain boundary precipitation of chromium-rich phases.
The attainable combination of strength and stress crack corrosion resistance is limited by the fact that the structure-forming forms required for these two properties are formed at distinctly different tempering temperatures, i.e.:
The invention seeks to avoid these disadvantages. It has as its object to provide a martensitic hardenable steel which has an improved combination of strength, ductility and corrosion resistance, and also a heat treatment process for such an alloy.
A transformation controlled nitride precipitation hardening heat treatable steel has the following composition (data in wt. %): 15-18 Cr, max. 0.5 Mn, 4-10 Ni, max. 15 Co, max. 4 W, max. 4 Mo, 0.5-1 V, at least one from Nb, Ta, Hf and Zr totaling between 0.001 and 0.1, 0.001-0.05 Ti, max. 0.5 Si, max. 0.05 C, 0.13-0.25 N, max 4 Cu, rest iron and usual impurities, under the condition that the weight ratio of vanadium to nitrogen, V/N, is in the region between 3.5 and 4.2.
By the choice of the alloying elements, a high corrosion resistance can be attained, besides a high strength and ductility. It is appropriate if the steel has 1-10 wt. % Co; 0.5-3, preferably 0.5-1.5, wt. % Cu; 15-17, preferably 15.5-16.5, wt. % Cr; 0.5-0.7 wt. % V, 0.16-0.2 wt. % N; 0.01-0.07 wt. % Nb, and a total of Mo and W in the range 1-6, preferably 1-4.
Preferred Mo contents lie in the range of 1.5-3 wt. %; of Mn, in the range of 0.02-0.4 wt. %; and of Si, in the range of 0.02-0.25 wt. %. The C content is preferably 0.02 wt. %.
It is furthermore advantageous if the alloys according to the invention are heat treated as follows: Solution annealing at 1,050-1,250xc2x0 C./0.2-10 h, preferably 1180xc2x0 C./2 h,; cooling in air to RT; intermediate annealing at 640xc2x0 C.-780xc2x0 C./0.2-10 h, preferably 2 h; tempering treatment at 570-630xc2x0 C./0.2-5 h, preferably 600xc2x0 C./1 h. Within the scope of this heat treatment, an increased volume proportion of tempering austenite is produced, and the special nitrides are not only used for grain size limitation at high austenizing temperatures and for precipitation hardening, but also make possible a finer distribution of austenite fractions within the martensitic base structure.